1. Field of the Invention
The present invention relates to a communication timing control method, a communication timing control apparatus, a node, and a communication system, more particularly to a method of avoiding data transmission collisions and reducing delays in multi-hop communication.
2. Description of the Related Art
Known methods by which a plurality of spatially distributed nodes can transmit data without collisions include time division multiple access (TDMA) and carrier sense multiple access (CSMA), the latter including carrier sense multiple access with collision avoidance (CSMA/CA) and carrier sense multiple access with collision detection (CSMA/CD) . A discussion of these methods can be found in, for example, Waiyaresu LAN Akitekucha (Wireless LAN Architecture), edited by Matsushita and Nakagawa, Kyoritsu Shuppan, 1996, pp. 47, 53-59, and 69 (in Japanese).
In the CSMA methods, a node with data to transmit determines whether other nodes are transmitting by sensing their carrier signals, and waits until no other node is transmitting before transmitting itself. The CSMA methods have the disadvantage of severely restricting the number of channels that can be used simultaneously.
In the TDMA method, different time slots are assigned to different nodes, and each node transmits data in its own assigned time slot. TDMA can provide more simultaneous communication channels than CSMA, but when the set of communicating nodes changes dynamically, an administrative node must reassign the time slots dynamically. A weakness of the TDMA system is that if the administrative node malfunctions, the entire communication system may be brought down. The process by which time slots are assigned dynamically to nodes is also complex, making it difficult to respond promptly to changing conditions. A further problem is that the width of the time slots cannot be changed.
Another problem with TDMA is that each node must await its time slot before it can transmit. In multi-hop communication, accordingly, a delay occurs at each node on the communication path. As the number of hops increases, the total delay can become objectionably large.
It would be desirable to have a more flexible method and apparatus for controlling communication timing, so that each node could communicate effectively without having to receive timing control instructions from an administrative node. It would also be desirable to reduce the delay in multi-hop communication.